A very beautiful and complex process. In very few words, initially there are a bunch of undifferentiated cells. They are all identical and without a particular function assigned to them. They are called stem cells. They start dividing in more cells, and with time, the new cells start being different, and with an specific function in our body.
Please, visit the web site of Stem Cell Information of the national Institute of Health for more information.

April 16, 2009

jacean (age 13, United States,New York)

okay iv always wonder since i was a baby and now im 13 and still no answer okay what are we made of okay i know where made of cromazone sbut what are they amde of and wahats that made of do u get me basucle what are humans made of

Humans, animals and EVERYTHING in the universe are made of elemental particles, which when combined create atoms, molecules, etc.

March 4, 2009

Mark (age 56, Australia)

Dear Scientist, Richard Dawkins, in his book ,The God Delusion, states that every atom of the human body is replaced over a period of time. On average, how long does it take for this to happen? Thank you.

There are enzymes released when a raw onion is sliced. These enzymes react with other substances released to form a sulfur compound, and the gas formed then reacts with the water in your eye to produce sulfuric acid.

February 8, 2009

David (age 14, England)

DO HUMAN BEING LIVE DURING THE TIME OF THE DINOSAURS

The Mesozoic Era (248 to 65 million years ago) was the Age of Reptiles; dinosaurs flourished and then went extinct. Mammals, birds, and flowering plants also appeared for the first time. However, Homo Sapiens first appeared about 200 000 years ago.

February 3, 2009

Rosita (age 13, H-town)

Why is their life on earth

It is called The Goldilocks Theory: Just as Goldilocks found the porridge that was just right, the Earth seems to be just right for living creatures. The Earth seems to be the perfect distance from the sun for lots of water.

In few words, there is a lot of salt in the human body. Salt can be found in every cell in our bodies, reaching an average of about 250 grammes (a cupful) for an adult. This is way our tears and sweat are salty.

September 11, 2008

cya (age 13, india)

there is a girl in our school who is totally white,her eyebrows and eyelashes are white and her hair are blond. she cannot bear the sun's heat. some children say that once she drank milk immidiately after eating fish and now her cells do not work. sir, could you please tell that what is the disease she is having and how can we cure it?

The student in your school is a case of albinism, characterized by a partial or total lack of melanin pigment in the eyes, skin and hair. Albinism is hereditary; it is not an infectious disease and cannot be transmitted through contact, blood transfusions, etc. It is not due to any kind of food ingetion.

July 2, 2008

Zach (age 9, FL,USA)

What Native Americans are there?

Adapted from the Encarta Enciclopedia article Native Americans of North America:
Most Native Americans in the United States belong to federally recognized tribes. There are more than 550 such tribes, including more than 220 village groups in Alaska. The tribes vary enormously in size. At the time of the 2000 census, the only tribes with more than 100,000 people were the Cherokee, Navajo (Diné), Sioux, and Chippewa. Most tribes had populations of less than 10,000, and several California tribal bands had only two to three members.

May 14, 2008

megan (age 9, bahamas)

how do people live in the tundra?

The average winter temperature of Siberia and the sub arctic sections of central Asia, Canada, and central Alaska is about -20 F. Under this cold condition, people have to dress well and to have certain resources available. For example, they live in small communities with stores, churches and schools. They also eat a somehow different food, like seal, bear, fish, geese, etc.

February 8, 2008

Pascha (age 12, New Zealand)

Why does life exist on earth?

Just as Goldilocks (the character in the children tale) found the porridge that was just right, the Earth seems to be just right for living creatures. The Earth seems to be the perfect distance from the sun for lots of water. For more about this topic, visit our web page The Goldilocks Theory.

December 21, 2007

Christine (age 34, Il, United States)

How often are new body cells made?

Cells on the surface of the human skin rub and flake off steadily and are continuously replaced with new ones. About every 30 days, the human body produces a totally new skin, that is almost 1 000 new skins in a lifetime. But not all types of cells are replaced, once brain cells die, new ones are not created.

December 10, 2007

candy (age 28, philippines)

which contributes more to the formation of the child?the sperm or the egg cells?

Actually, the contribution from both parents is even, the mother contributes half of the hereditary material to the child and the father contributes the other half.

July 13, 2001

Heather (56, New Zealand)

Approximately how many people have ever lived on Earth?

I think the figure is usually estimated at 60 billion, which means that for every 1 person living now, 9 others have lived before.

October 9, 2000

Dan (age 19, Colorado, USA)

At what rate are humans populating the Earth?

Most people would say that the world's population is growing at an exponential rate. Those concerned about resources would say that the world's population is growing at an alarming rate. The U.S. Census Bureau has an estimation of the world's population that is in real-time. Between the time I looked yesterday and looking today, the world's
population increased by almost 250,000 people. I'd can't say for sure whether that's exponential because I haven't looked for more than a day, but it is alarming!

August 4, 2000

Myleen (age 15, Philippines)

What is the importance of metals to industry and the human body?

Metals are obviously important to all sorts of industry, because very few materials have the same durability at low cost.

Many metals are very important to the human body because they're required as cofactors in many enzyme reactions. Fe, Cu, Co, Mg, and Zn, to name a few, are all important cofactors without which our metabolism wouldn't work.

May 10, 2000

Ilya (11, USA)

How many people are on this
planet!?!

The number is always changing, but the latest estimates say there are over 6 billion people on this planet today. Whew, that's a lot of people!

February 14, 2000

Geoff (age 13, Wisconsin, USA)

What is the scientific name for the fear of the moon?

Selenophobia--and fear of the Sun is heliophobia.

April 11, 2000

Carla (age 44, North Carolina, USA)

It says in the bible that, God created Adam ( which is a
man) first. I believe that, but what is puzzling me is,
who was first the cavemen or Adam? can you answer
that one?

I can't really answer that one for sure, but I would have to say that caveman probably came first.
But that doesn't mean the Bible is false. Scientists are now saying that neanderthals, or cavemen, aren't human at all. Evidence has shown that humans and neanderthals have very different DNA.
So, I guess we dodged your bullet this time! Of course, scientists are still arguing this one, so it may take a while before we can settle this debate!

March 3, 2000

Kyle (age 17, Ontario, Canada)

When I eat food, I turn some of it into energy and heat. How
does that energy turn back into matter? Is the universe
getting less mass, and more energy?

When we eat food, our body uses the energy found in the chemical bonds, but we can't convert any of the matter we eat directly into energy (this only happens in a nuclear reaction).

When we use this energy, we do convert some of it to heat and mechanical energy, and this energy is lost to our environment (for example, the warm chair I'll get up from in a few minutes). This energy is never really recovered.

February 8, 2000

Valerie (age 11, Washington, USA)

Why does your skin get wrinkly after you go swimming?

The wrinkling happens because the thicker skin on our hands and feet soaks up water. The swollen keratin (the thicker, callused part) causes the wrinkling. Notice that the thinner skin on most of the rest of your body doesn't get wrinkled, only the callused parts.

December 24, 1999

Wanda(age 39, Pennsylvania, USA)

What vitamins does the sun help humans produce?

Ultraviolet light from the sun changes a form of cholesterol in your skin to vitamin D. Only about 15 minutes a day in the sunlight is necessary. You can also get vitamin D from fortified dairy products, and in lesser amounts from tuna, salmon, and fortified breakfast cereals.

November 16, 1999

Robert (age 13, California, USA)

Why can't we breath in space?

Humans need oxygen to breathe. In outer space, there is essentially no oxygen at all--thus no breathing!

August 16, 1999

Leah (age 21, Michigan)

If you were to bleed in outer space, would the blood be blue or red?

Blood is normally a bluish color, but turns red when oxygen is present. That's why your blood looks red when you bleed! In space, there isn't any air, which means you blood should be blue if you get cut.

June 1, 1999

Sunday (age 16, Michigan, USA)

How is it that the sun can give people energy?

The sun can give people energy in a couple ways. When sunshine warms your body it is giving you some heat energy through radiation. The most important way the sun gives us energy is through the food we eat. Plants can convert the suns energy into food for itself, which we can eat. In fact, all the food we eat relies somehow on the sun. Kinda makes me hungry.

May 24, 1999

Adam (age 8, Pennsylvania, USA)

Do humans itch in space?

Humans do itch in space. Itching is a reaction to irritation of the skin, caused by insects, dust, air, etc. Although there aren't any mosquitoes in space, other stuff can cause itching. Astronauts have a lot of trouble scratching their noses and ears during a space walk. Their helmet gets in the way!

May 21, 1999

Lester (age 69)

Why are full siblings (same mother & father) only 50% related genetically, when you would think that having the same parents they would be 100% related?

The 50% is not a strict rule, just a general average, and it does make sense with the way that fertilization occurs. Each sperm and each egg have exactly half the number of chromosomes of normal cells. Which half is randomly determined for each cell. During fertilization this genetic material combines to complete one full set. This method ensures that each parent passes on some of their traits to the next generation without doubling the number of chromosomes present. You can see how that would be a problem.
There are 23 pairs of chromosomes in humans, if the number was not cut in half in eggs and sperm, the second generation would have 46 pairs of chromosomes, and the number would double in each successive generation, could get messy.

May 18, 1999

Shannon (age 13, Pennsylvania, USA)

What would it take (or what would we need) to live on Uranus?

Well, first of all, the atmosphere of Uranus is mostly methane gas. So we would need to transport and store a lot of Oxygen to breathe! Also, the average surface temperature is -200 degrees
C. We would need some really huge heaters to keep warm!

March 17, 1999

Tom (age 30, California, USA)

Why do some people - including me - sneeze in the sun?

This is actually caused by a strange alergy called allergic conjunctivitis, which involves inflamation of the lower eyelid. The sneezing is caused by a reaction to the ultraviolet rays from the sun!

March 8, 1999

Stephanie (age 11, Korea)

How do optical illusions effect our eyes and brains? and what other things are there about optical illusions?

Optical illusions occur when the brain misinterprets visual signals. The brain tries to recognize what it sees...and sometimes it can be tricked into "recognizing" things that aren't there.

October 15, 1998

Kyle (Age 7, North Carolina, USA)

How does air make us survive? How does air change temperature?

Well, for starters, humans get the oxygen we breathe from the air! Also, the air forms an atmosphere around the Earth, which protects us from harmful rays from the Sun. Air heats up when energy (from the
Sun, for example) causes the molecules of air to move faster. It cools down when energy leaves the molecules and goes into something else, like the ground.

September 1, 1998

Bobby (Age 13, Canada)

Is the world going to end in the year 2000?

Although a lot of people believe it will, we took a poll here in our lab - 100% unanimous vote that the world won't end in the year 2000. It would just be too predictable!.

June 26, 1998

Breona (Age 13, California, USA)

How do humans benefit from mosquitoes?

I'll bet the last time you were bitten by one you probably thought they
were good for nothing! Well, from a human's perspective, you're just
about right. Mosquitoes can spoil an otherwise pleasant summer evening
by their persistant harassment of us in an effort to suck our blood. And
in some parts of the world, mosquitoes can be dangerous because they can
carry deadly diseases, like malaria. But did you know that only female
mosquitoes bite? Male mosquitoes sip
nectar from flowers and are completely uninterested in your bare skin.
In fact, some species of plants, including an endangered orchid, depend on
mosquitoes for their pollination. Mosquitoes are also important to some
animal species as food. Frogs, lizards, birds and bats are all quite
fond of mosquitoes for snacking. In fact, a single little brown bat can catch and eat 600
mosquitoes an hour! My advice: if you don't like mosquitoes, make
friends with a bat!!

June 22, 1998

Genevieve (Age 13, New Jersey, USA)

I've heard that in the future, the Sun will turn into a red giant and swallow the first three planets (Earth being one of them). I know this will happen in about 5 or 6 billion years but is there any chance of life on earth moving to another location
? What do you suppose will happen?

If humans are still around then, I think it would be completely reasonable to think that they could move to a different location. Some of the outer moons of Jupiter and Saturn would be a lot warmer then. They might even have liquid oceans at that po
int. My guess is that Earthlings will have figured out faster forms of travel by then so they could go to a completely different solar system...knowing how much has been accomplished in just 100 years on Earth, things beyond our imagination will be d
one in 5 billion years!

March 27, 1998

Woody (age 11, Florida, USA)

If you stay in space for too long, does your heart beat slower because it has to work harder?

Actually, in space your heart would beat slower because it would have to work less, not more. On Earth, your heart has to work against gravity pulling your blood to your feet. In space, since there is no gravity, your heart wouldn't have to work as
hard. Because it wouldn't have to work as hard, your heart would beat more slowly and eventually get very weak.

March 13, 1998

Lauren (age 8, Missouri, USA)

Why do people blush?

People blush when blood rushes to just under their skin. Since the blood is red, it causes the face to look red also. This usually happens when people are embarassed. It happens more often to some people than to others.

December 8, 1997

David

Will we be able to live on other planets?

Probably. Mars is the most likely planet humans will try to colonize first since it is more similar to Earth than any of the other planets. NASA plans to send a manned mission to the Red Planet by the year 2020.